DESCRIPTION: DNA tumor viruses represent proven powerful tools for dissecting mechanisms responsible for the development of human cancers. Human adenovirus type 9 is unique among tumorigenic adenoviruses in causing exclusively estrogen-dependent mammary tumors in rats and in having E4 region-encoded open reading frame 1 (E4-ORF1), rather than E1 region-encoded E1A and E1B, as its primary oncogenic determinant. Our results indicate that the tumorigenic potential of E4-ORF1 depends on its ability to complex with a select group of cellular PDZ proteins (MUPP1, MAGI-1, ZO-2, and DLG) at the plasma membrane and in the cytoplasm. Significantly, these PDZ proteins, which typically function as adaptors/scaffolds in signal transduction, are common cellular targets for the human T-cell leukemia virus type 1 Tax and high-risk human papillomavirus E6 oncoproteins. We also recently reported that E4-ORF1 selectively stimulates cellular phosphoinositide 3-kinase (PI3K) at the plasma membrane by a novel PDZ protein-dependent mechanism(s). Evidence also suggests that some E4-ORF1-associated PDZ proteins may likewise function as tumor suppressors and, consistent with this idea, E4-ORF1 aberrantly sequesters MUPP1, MAGI-1, and ZO-2 in the cytoplasm of cells. These observations support the existence of two separate cellular pools of the PDZ proteins, functioning either to regulate signal transduction at the plasma membrane or to suppress inappropriate cell proliferation. Consequently, we hypothesize that interactions of E4-ORF1 with its cellular PDZ-protein targets act not only to promote constitutive PI3K activation but also to block putative tumor-suppressor activities. The goals of this application are (Aim 1) to reveal tumor-suppressor functions and mechanisms for the PDZ-protein targets of Ad9 E4-ORF1 and (Aim 2) to determine the molecular mechanism for PDZ protein-dependent stimulation of the PI3K pathway by E4-ORF1. In the long-term, the knowledge gained by completing the proposed work is expected to contribute to the development of new therapeutic strategies for preventing many human malignancies.